The operation of thermal devices depend on the temperature distribution inside the thermal device. Temperature can be measured e.g. by inserting thermal sensors in various locations in the thermal device. However, the maintenance of multiple temperature sensors is a significant burden for the operator of the thermal device. The thermal device may refer e.g. to a boiler, a pyrolysis reactor, a torrefaction reactor, or a gasifier.
Prior art solutions include also some acoustical and optical methods for determining temperature inside a thermal device. However, these methods can not as such, by using only a single opening, be used to measure a profile of temperature and/or molecular number density inside a thermal device. Moreover these methods can not as such be used to measure a three dimensional profile of the aforementioned quantities.